Planets Venus & Neptune to Appear Side-by-Side on Friday

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On Friday evening (Jan. 13), skywatchers will have the chance to
see the brightest and faintest planets in the solar system next
to one another as they glide across the sky.

The
bright planet, Venus, has been a brilliant "evening star" for
a couple of months now, and will continue to grow brighter until
it reaches its maximum on April 30. Meanwhile, dim Neptune has
been moving along on the outer edge of the solar system.

Astronomers measure the brightness of objects in space using a
scale called magnitude. On this scale, the lower an object's
magnitude is, the brighter it appears in space. Objects with
negative numbers (like Venus) are extremely bright.

On Friday, Venus will have a magnitude of –4.03 while
Neptune will be +7.95, a total difference of 11.98
magnitudes. What does that mean? A difference of 5 magnitudes on
this logarithmic scale equates to 100 times difference in
brightness, so a difference of 12 magnitudes amounts to about
100,000 times difference. So Venus will be 100,000 times brighter
than Neptune.

Very interesting on paper, but can you actually see this
event in the sky?

For that you need some planetarium software. The time of
conjunction is 0700 UT (or GMT); that's 2 a.m. in EST, and Venus
would have set long before. [ January
2012 Skywatching Events ]

Check your planetarium software (if you have it) around 7 p.m. on
Thursday or Friday (Jan. 12 and 13), the dates on either side of
the event, to see which will give the closest distance between
Venus and Neptune. On Jan. 12, they will be 1 degree 20 arc
minutes apart, and on Jan. 13 the separation will be about 1
degree 9 arc minutes, so Jan. 13 will be the better date.

As a reminder, your closed fist held out at arm's length covers
about 10 degrees of the night sky, so Venus and Neptune will
appear substantially closer than your fist.

Now the challenge is to find the best time to observe
the event.

You will need a low western horizon. Neptune will be very faint,
around 8th magnitude, so you need a time late enough that it will
be visible, but not so late that it will be too low in the sky.

The scientific way is to check the altitude of the sun: –6
degrees is the end of civil twilight, –12 degrees is the end of
nautical twilight, and –18 degrees is the end of astronomical
twilight, the one we want here. At my location, the Sun will
reach –18 degrees altitude at 6:46 p.m.

All the indications are that the optimum time to observe this
close conjunction of bright and faint will be around 6:45 p.m. on
Jan. 13 for my location. In this case, your
mileage will definitely vary, because twilight is very sensitive
to the observer's location.

So, you will need to use your planetarium software to
find the optimum time for your location. It will be
interesting to see how it varies from one place to another.

If you have an amazing skywatching photo you'd like to share for
a possible story or image gallery, please contact managing editor
Tariq Malik at tmalik@space.com.